This invention relates to a process for the production of semirigid foams containing urethane groups which exhibit very good flow properties during the foaming process. Due to the good flow properties of the reaction mixtures, these reaction mixtures are even capable of filling closed molds of complicated construction including, for example, molds of the type used in automobile manufacture for the production of back-foamed instrument panels, without any unwanted defects and voids.
German Offenlegungschrift 4,001,556 describes mixtures of tolylene diisocyanate and diphenylmethane diisocyanate as the polyisocyanate component for the production of flexible foams. Unfortunately, these flexible foams are unsuitable for the production of instrument panels because they do not show satisfactory flow properties in the production of semirigid polyurethane foams. In addition, tolylene diisocyanate is often undesirable to processors for reasons of industrial hygiene.
It has now surprisingly been found that distinct improvements in the flow properties of foamable reaction mixtures can be obtained wherein the reaction mixtures comprise special mixtures of diphenylmethane diisocyanate (MDI) and polyphenylene polymethylene polyisocyanate (PMDI). These MDI/PMDI mixtures differ from hitherto known mixtures in their relatively high content of monomeric MDI isocyanates, more particularly the 2,4'-MDI content. Typical modifications of these isocyanates, for example, by urethane, carbodiimide, isocyanurate or urea groups, lead to similar positive effects.